Why we observe only retarded gravitational waves, not advanced?

Jarek Duda

Jarek Duda

Registered Senior Member
General relativity is rather solved in time symmetric way, like the least action principle condition in Einstein's field equations, what as in e.g. Wheeler-Feynman absorber theory requires symmetrically both retarded and advanced solutions.

So why seems there are only considered retarded gravitational waves?

Can we exclude being advanced wave for all observed events ( https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations )? If not, should they use original chirp shapes, or maybe time reversed?
 
ps. made a diagram - assuming Big Crunch and solving by the least action principle, evolving backward there should be also formed black holes, which collisions for us would be advanced gravitational waves with reversed chirps.

It would be worth searching for such reversed chirps in LIGO historical data - probably getting negative result, still allowing for valuable article - starting discussion and the search for advanced gravitational waves.

If you would know somebody interested in collaboration on such article, I would gladly help ...

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Gathered some more arguments.
As the main source of gravitational wave events is just orbiting of e.g. two black holes, and evolving toward minus time orbiting remains orbiting, so using Euler-Lagrange toward minus time (t -> -t), or the least action principle, there should be generated similar waves - for us being advanced of similar chirp shape as retarded. LIGO just measures lengths - invariant to time symmetry, so should see both retarded and advanced waves.

Therefore, maybe some of current ~300 events ( https://catalog.cardiffgravity.org/ ) could turn out advanced? Some arguments:

- ultimate confirmation should be certain lack of (retarded) EM counterpart when required (e.g. neutron star merger), still only 1 per ~300 observed, leaving advanced wave possibility (?),

- some events are believed to happen too early, like 66 + 85 -> 142 merger starting in 50-120 black hole Mass Gap, e.g. https://www.symmetrymagazine.org/ar...st-scale-could-explain-impossible-black-holes - advanced would have more time,

- pulsar arrays show vibrations of the Universe requiring more than expected orbiting supermassive black holes - https://theconversation.com/to-map-...uilt-a-detector-the-size-of-the-galaxy-244157 - advanced could add them,

- the largest observed luminosity distance is ~27Gly: twice the age of the Universe - maybe it is worth to consider advanced?

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Is this an image of how you think?

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Are we meant to decipher this? They are blocks of loosely-related factoids, mashed together as close as possible. But "mashed together" does not have a well-defined operation (such as say, "this follows from that") in any form of communication I am aware of.

If you are trying to communicate with others, isn't it emcumbent on you to communicate in a language that's common and comprehensible?
 
The central in diagram is standard GW picture from orbiting e.g. black holes - emitting gravitational waves for forward evolution (Euler-Lagrange), but we can also reverse time there t -> -t getting backward evolution governed by the same equations, also with just orbiting objects ... why against e.g. https://en.wikipedia.org/wiki/CPT_symmetry only in one perspective it should emit gravitational waves?

Then there are listed some current problems, which just disappear if accepting advanced waves (currently neglected).

I can elaborate if you have a more specific question.
 
Extended it to https://arxiv.org/pdf/2512.20692

Symmetry allows both retarded and advanced solutions - their choice should depend on the boundary conditions, like now having more absorbers than emitters.
Wheeler-Feynman assumed 1/2-1/2 symmetric contributions ... but any convex combination is allowed, and current assumption of 1-0 only retarded should be at least verified experimentally, especially that while there is now more absorbers than emitter, this imbalance might not be so perfect.

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Got 14-day arxiv trending in https://www.emergentmind.com/ and animation:
 
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Jarek Duda said:
emitting gravitational waves for forward evolution (Euler-Lagrange), but we can also reverse time there t -> -t getting backward evolution governed by the same equations, also with just orbiting objects
Click to expand...
Have you thought about what this physically means? It would mean the advance wave field carries energy-momentum towards (not away from) the source, so the orbiting objects would get further apart over time. Do you really consider this a physically reasonable boundary condition?
 
Indeed time symmetry switches absorption of retarded wave, with emission of advanced wave.
Hydrodynamical analog is great to get intuitions: wave behind marine propeller carries energy, momentum, and angular momentum - like photon ... and we can practically apply time symmetry by just reversing its rotation direction - getting "pulling photons" able to pull energy from excited resonator (by stimulated emission).
Going back to EM, observation of such time reversed "pulling photons" would need initial excitation of telescope sensor - usually avoided by its cooling.

But LIGO just measures lengths - which are T/CPT invariant, so in theory should see both retarded and advanced waves - and there are arguments there might be already some advanced among ~400 GW observations ( https://arxiv.org/pdf/2512.20692 ), like lack of (retarded) EM counterpart, events too early to happen according to current knowledge, or missing orbiting SMBH for observed "vibrations of the Universe" if considering only retarded.

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Maxwell equations allow retarded, advanced solutions and their convex combinations - which should be chosen based on the boundary conditions, I would say mainly imbalance between emitters and absorbers, but maybe entropy is indeed also involved (?)
Anyway, while clearly there is such imbalance, the big question is if it is perfect?
In other words, what is the status of perfect 1-0 only retarded currently default assumption? Why not e.g. 0.9999-0.0001 instead?
I don't know any theoretical argument (?) nor experimental confirmation (?), so maybe instead of just assuming, it is worth to finally verify it from data?

And gravitational data brings many types of suggestions for advanced, like lack of (retarded) EM counterparts, "too early events" for retarded, or missing SMBHs for observed PTA "vibrations of the Universe" if considering only retarded SMBHs ( https://arxiv.org/pdf/2512.20692 )

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In general relativity we live in 4D spacetime minimizing action, in QFT in Feynman ensembles of 4D scenarios ...
Both use eternalism/block universe philosophy of time ( https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) ) - traveling through static 4D solution, also are CPT symmetric ( https://en.wikipedia.org/wiki/CPT_symmetry )
Knowing that, please explain where exactly asymmetry of Maxwell solutions come from?

This is a big question, called Asymmetry of Radiation ( https://link.springer.com/article/10.1007/BF00733218 ), requiring asymmetry in boundary conditions, like now having more absorbers than emitters ...
This imbalance is essential, but not necessarily perfect as assumed - beside faith expressions like above, science differs from religion by experimental verification.

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Mainstream says there have left only unification of GR with QFT of Standard Model ... both are T/CPT symmetric, work on 4D scenarios, use Einstein's block universe philosophy of time.
As asymmetries cannot be in equations, they need to be in solution - like throwing a rock into a lake symmetric in equations ... or Big Bang when everything was localized, hence entropy was low - creating entropy gradient we call 2nd law of thermodynamics, also anchoring our reason-result relation chains.

But Asymmetry of Radiation seems essentially different - in solutions of e.g. Maxwell equations, dependent on the boundary conditions, but instead of some abstract entropy, photon exchange needs both emitter and absorber e.g. in S-matrix <psi_f |U| psi_i> - amplitude of such event should depend on presence of emitters/absorbers ...
... and while there is definitely their imbalance, it doesn't need to be so perfect as assumed 1-0 only retarded - this guess should be finally verified, and e.g. LIGO should allow for that by just including advanced into considerations.
E.g. for each event trying to estimate probability of being advanced (like 0 if present EM counterpart, higher for "too early events"), also including in e.g. cosmological models as additional asymmetry parameter - to be estimated from data, testing currently used hypothesis of being zero.
 
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